Shannon Matthews has been missing for three weeks
More than half of the UK's specialist search dogs are involved in the hunt for missing schoolgirl Shannon Matthews, police have said.
It is three weeks to the day since the nine-year-old was last seen leaving Westmoor Junior School in Dewsbury.
West Yorkshire Police said the operation was the biggest missing person's case it had undertaken since the hunt for the Yorkshire Ripper.
A national newspaper is offering a £50,000 reward for information.
The Sun has more than doubled its previous offer of £20,000 in the hope of a breakthrough.
A huge search by police officers has failed to find any trace of Shannon.
Ch Insp Graham Armitage, who is in charge of the search teams, said 16 of the UK's specialist "victim recovery" dogs, trained to find blood and human remains, were now being used.
He said the dog teams had come from as far away as Hampshire and Glasgow.
He said handlers were working 14-hour days to maintain the intensity of the search.
Mr Armitage said: "It's the biggest investigation of its kind now, it's certainly the biggest missing persons inquiry since the Yorkshire Ripper which I also worked on."
Dog teams from as far away as Glasgow are helping the search
The officer was speaking in Crow Nest Park, about a mile-and-half from Shannon's home in Moorside Road, Dewsbury Moor.
Search dogs from the Surrey, Hampshire, Strathclyde and Greater Manchester forces joined local officers in the park on Tuesday and searching in neighbouring properties.
Mr Armitage said: "There is a massive amount of resources committed here. It is a huge operation.
"Every individual here is committed to finding Shannon.
"We would obviously like to find Shannon alive but we've got to consider all options with the search."
Asked whether the search would extend beyond Dewsbury and the immediate area, he said the operation could only be guided by information it had been given.
"It's all about information," he said. "When we get that information we go where it takes us."
Det Supt Andy Brennan, who is leading the overall search for Shannon, repeated his statement that he is "gravely concerned" for Shannon.
He said officers were about two-thirds of the way through a list of 3,000 residential and commercial premises which were being thoroughly searched.
A sponsored walk is due to take place in Dewsbury later to raise money to help in the search for the youngster.